Pattern transfer with self-similar sacrificial mask layer and vector magnetic field sensor

ABSTRACT

A method is provided for producing a lithographic pattern using a mask that includes the same materials as the material to be etched, allowing the pattern to be transferred and the etch mask to be removed in one step. In accordance with features of the invention, the method includes building up of a layer or layers of material of specific thickness on top of a substrate so that temporal control of an etching process allows formation of the desired pattern. Different exchange bias directions can be established by the use of shape anisotropy for the exchange biased component of a spin valve device. This enables several different magnetic reference directions to be present on a single chip, which allows a more compact magnetic field sensor to be developed. In accordance with features of the invention, different field directions are established on one single chip by using shape anisotropy

This application claims the benefit of U.S. Provisional Application No.60/523,590, filed on Nov. 20, 2003 and U.S. Provisional Application No.60/523,596, filed on Nov. 20, 2003.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the United States Government andArgonne National Laboratory.

FIELD OF THE INVENTION

The present invention relates to a method for producing a lithographicpattern using a mask which consists of the same materials as thematerial to be etched, allowing the pattern to be transferred and theetch mask to be removed in one step and a vector magnetic field sensordefined by a single chip sensor upon which different magnetic referencedirections have been established that allows the measurement of thedirection and magnitude of an external magnetic field.

DESCRIPTION OF THE RELATED ART

One of the major steps in the manufacture of nanometer scale devicesinvolves the transfer of a pattern into a multi-layered thin filmstructure. The most commonly used method of doing this involves severalsteps. First, an etch mask using a lithographic process is generated andapplied to the appropriate substrate. Then the pattern is transferred tothe desired substrate, often by directional etching. At this point, inmany cases the mask must be removed using a selective chemical etchingprocess. As the material and chemicals used in this step need to beoptimized for every material in the multilayered film, choice ofsuitable reagents can be a formidable task.

Some of the most sensitive magnetic field sensors available today arespin-valve devices. In such devices a reference magnetic field directionis established through exchange biasing due to the coupling between aferromagnet and an antiferromagnet. Ordinarily, the reference directionof the exchange biased component is determined during the manufacturingprocess, either due to magnetic field cooling or due to preparationwithin a magnetic field, and is unidirectionally fixed for each chip.Therefore in order to measure the different components of a magneticfield, sensors having separate chips with different magnetic referencedirections must be used.

Important objects of the present invention are to provide a method forproducing a lithographic pattern using a mask which consists of the samematerials as the material to be etched, allowing the pattern to betransferred and the etch mask to be removed in one step and an improvedmechanism to measure the different components of a magnetic field.

SUMMARY OF THE INVENTION

In brief, a method is provided for producing a lithographic patternusing a mask that includes the same materials as the material to beetched, allowing the pattern to be transferred and the etch mask to beremoved in one step.

In accordance with features of the invention, the method includesbuilding up of a layer or layers of material of specific thickness ontop of a substrate so that temporal control of an etching process allowsformation of the desired pattern.

In accordance with features of the invention, different exchange biasdirections can be established by the use of shape anisotropy for theexchange biased component of the spin valve device. This enables severaldifferent magnetic reference directions to be present on a single chip,which allows a more compact magnetic field sensor to be developed. Asingle chip sensor upon which different magnetic reference directionshave been established allowing the measurement of an external magneticfield defines a vector magnetic field sensor of the invention.

In accordance with features of the invention, different field directionsare established on one single chip by using shape anisotropy.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiments of the invention illustrated inthe drawings, wherein:

FIGS. 1A, 1B, 1C, 1D, 1E, and 1F illustrate magnetic hysteresis loops ofline patterns measured with magneto-optic Kerr effect (MOKE) using anoptical cryostat;

FIGS. 2A, 2B, 2C, and 2D and FIG. 3 illustrate calculated hysteresisloops for a theoretical coherent rotation model;

FIG. 4 illustrates an exemplary etch-mask that mimics layers to bepatterned for producing a pattern transfer in accordance with thepreferred embodiment;

FIGS. 5-8 illustrate an experimental demonstration of a method forproducing a pattern transfer in accordance with the preferred embodimentand an exemplary demonstration sample for a vector magnetic field sensorin accordance with the preferred embodiment; and

FIG. 9 illustrates an exemplary design for a vector magnetic fieldsensor in accordance with the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The magnetic behavior of Fe lines on top of a continuous FeF2antiferromagnetic layer was investigated as a function of theorientation of the lines with respect to the applied magnetic field anda unidirectional anisotropy established by field cooling. Theorientational dependence of the asymmetric loop shift, called exchangebias, shows that the competition between shape and unidirectionalanisotropies modifies the exchange bias and the coercivity. Remarkably,in certain cases, exchange bias can be observed even when the appliedfield is perpendicular to the unidirectional anisotropy. Numericalsimulations with a coherent rotation model illustrate a rich phasediagram, which originates from the noncollinearity of the involvedanisotropies. Using this phase diagram, exchange bias and coercivity canbe predictably tailored. In particular, different preferredmagnetization directions can be designed in separately patternedstructures of the same sample with identical preparation and magnetichistory.

Although the role of shape anisotropy in homogeneous magnetic materialshas been well understood for a long time, it is shown here that addingshape anisotropy to magnetic heterostructures can give rise to anunexpected behavior due to a competition between shape anisotropy andinternal interactions of the heterostructure. Examples ofheterostructures, which received much attention lately, areferromagnetic/antiferromagnetic exchange-coupled systems. The couplingbetween an antiferromagnet and a ferromagnet can give rise to an inducedunidirectional anisotropy in the ferromagnet, which is referred to asexchange bias. The main characteristic of this induced anisotropy is ashift of the hysteresis loop of the ferromagnet along the field axis.This unidirectional anisotropy stems presumably from the way theantiferromagnet orders in the proximity of a ferromagnet, but a detailedunderstanding is still missing. Regardless of the missing microscopicunderstanding, exchange bias has become important for manymagnetoelectronic applications, because it pins the magnetizationorientation of one ferromagnetic layer, which then serves as thereference layer in a variety of device structures, such as spin valvesand magnetic memory elements.

For applications, it is often necessary to pattern the heterostructuresinto a confined geometry. Thus the question of how patterning influencesthe magnetic behavior arises naturally. Up to now, studies ofexchange-biased antiferromagnetic/ferromagnetic wires have beenrestricted to cases with shape anisotropy either parallel orperpendicular to the applied magnetic field. These studies showed amodified exchange bias similar to nanostructured networks ofexchange-bias systems. However, there has been no systematic study ofthe role of the shape anisotropy orientation and no quantitativeunderstanding of these effects has yet been obtained.

In this work, the exchange bias of Fe lines on an antiferromagnetic FeF₂film was studied as a function of line orientation with respect tocooling and applied magnetic fields, but fixed with respect to the FeF₂crystalline orientations. The main result is that competition andnoncollinearity between unidirectional exchange coupling and shapeanisotropy can give rise to an unexpected magnetic behavior. This opensup a straightforward pathway to tailor both the magnitude and directionof exchange bias, which can be applied to any exchange-bias system. Wecompare the experimental results to numerical simulations obtained froma coherent rotation model. The simulations give rise to a surprisinglyrich variety of hysteretic behavior. The magnetic behavior dependsstrongly on the ratio and relative orientation between shape anduniaxial anisotropies. In particular, when the ratio is less than 1,large exchange bias is observed even with magnetic fields appliedperpendicular to the unidirectional anisotropy. This permits theintroduction of several different preferred magnetization directions inseparately patterned structures, independent from material specificparameters, even if they have identical magnetic history.

Using the new patterning technique, which is described in more detailbelow, we defined 300-nm-wide polycrystalline Fe lines on top of acontinuous quasiepitaxial (110) FeF₂ film grown on a MgO (100)substrate. The FeF₂ and Fe are 90 and 10 nm thick, respectively. The Felines have a periodicity of 500 nm and cover several 100×100-μm² areas,each with a different direction with respect to the MgO [010] direction.Since all the patterns are on one single chip, it is assured that thelocal exchange interaction between the Fe lines and FeF₂ film and themagnetic history (i.e., magnitude and direction of the cooling field)are identical for all patterns.

The magnetic hysteresis loops of the line patterns were measured withmagneto-optic Kerr effect (MOKE), using an optical cryostat. Thetransverse MOKE geometry is used under ˜45° incidence, which allows usto measure the magnetization component Mpar parallel to the appliedfield. The laser beam is focused down to 50 mm diameter, which enablesus to address each of the Fe-line patterns individually. Magnetichysteresis loops measured at room temperature for the patterned Fe linesalong various directions are consistent with a uniaxial shape anisotropyK_(u)=150Oe.

FIGS. 1A-1E show hysteresis loops measured with MOKE at 35 K for threepatterns with the lines −45° in FIGS. 1A, 1D, 0° in FIGS. 1B, 1E, and+45° in FIGS. 1C, 1F oriented with respect to the applied field duringthe hysteresis loop measurements. The applied magnetic field is parallelto the cooling field for in FIGS. 1A-1C, while it is perpendicular forin FIGS. 1D-1F. The directions of the applied field and cooling fieldwith respect to the lines are indicated to the right of each plot.

For measurements in the exchange-biased state, the sample is cooled fromroom temperature to 35 K in an applied field of 1.5 kOe. FIGS. 1A-1Cshow magnetic hysteresis loops after field cooling for three patternswith the lines oriented at −45°, 0°, and +45° relative to the coolingand the applied field. The resulting exchange bias is similar (HE ˜475Oe) for all three patterns and only the shape of the hysteresis loop issomewhat changed by the different shape anisotropies. Furthermore, asexpected, the hysteresis loops for the patterns rotated +45° or −45° areessentially identical, see FIGS. 1A and 1C.

As shown in FIGS. 1D-1F the situation is completely different as soon asthe patterns are rotated 90° clockwise after field cooling. Theunidirectional anisotropy is now perpendicular to the applied magneticfield and therefore one would naively not expect to observe any exchangebias. Indeed, for the pattern where the cooling field direction isparallel to the lines and thus along the direction of the uniaxial shapeanisotropy, the exchange bias is negligible compared to the other cases,see FIG. 1E. On the other hand, for the lines at 45° to both the appliedand the cooling fields, there is an exchange bias, see FIGS. 1D and 1F.However, note that the sign of the exchange bias is opposite for the twoorientations, even though the magnetic history is exactly the same.

It is instructive to compare these experimental results with numericalsimulations based on a coherent rotation model similar to earlier works.If we assume a homogeneous magnetization in the Fe lines, then the freeenergy f can be written as:f=HM _(s) cos θ−K _(E) COS (θ−θ_(E))−K _(u)(cos²(θ−θ_(u))where H is the applied field, M_(s) is the saturation magnetization, uis the angle of the magnetization with the applied field, K_(E) andK_(u) are the unidirectional exchange coupling and the uniaxial shapeanisotropy, and θ_(E) and θ_(u) are the angles between the applied fieldand these two anisotropy axes, respectively. Hysteresis loops aredetermined numerically via energy minimization of the above equation.Results are shown in FIGS. 2A-2D for different ratios of K_(U)/K_(E) andfixed values of θ_(u)=90° and θ_(u)=45°, corresponding to the case inFIG. 1D. As one can see, a range of hysteretic behavior can be observeddepending on the ratio K_(U)/K_(E).

FIGS. 2A-2D show hysteresis loops from the coherent rotation model withθ_(E) and θ_(u) fixed to 90° and 45°, respectively. Shown are thelongitudinal (solid line) and transverse (dashed line) magnetizationsM_(par) and M_(perp) normalized by the saturation magnetization. Thecurves are for K_(U)/K_(E) ratios of 0 in FIG. 2A, 0.3 in FIG. 2B, 0.95in FIG. 2C, and 1.5 in FIG. 2D. The solid symbols in FIG. 2B indicatethe average of the two hysteresis branches from FIG. 1D.

FIG. 3 shows calculated H_(E) (solid line) and Hc (dashed line)normalized by K_(E)/M_(s) and K_(c)/M_(s), respectively, as a functionof K_(U)/K_(E) at fixed θ_(E)=90° and θ_(u)=45°. The regions ofdifferent hysteresis behavior are indicated by I, II, and III.

The exchange bias H_(E) and the coercivity H_(c) values extracted fromthese simulated loops are plotted as a function of K_(U)/K_(E) in FIG.3. One can distinguish three types of behavior. For vanishing K_(u),H_(E) also vanishes and the magnetization simply rotates reversibly fromone direction to the opposite, whereby at remanence the magnetizationalways points along the unidirectional anisotropy K_(E), see FIG. 2A.With increasing K_(u) the magnetization still rotates reversibly, albeitasymmetrically, see FIG. 2B. This gives rise to an H_(E) which increaseslinearly with K_(u) see region I in FIG. 3. When K_(U)/K_(E) reaches0.85, the hysteresis loop shows irreversible behavior, see FIG. 2C.Notice that the exact value at which the irreversible behavior becomesimportant depends on the angle between the uniaxial and theunidirectional anisotropy. For K_(U)/K_(E) larger than 0.85, H_(c)increases and H_(E) decreases, see region II in FIG. 3 until they bothbecome close to K_(E)/2M_(s) near K_(U)/K_(E)=1. For K_(U)/K_(E)<1, theperpendicular component of the magnetization always points along thedirection of the unidirectional anisotropy during the magnetizationreversal. The situation changes completely at K_(U)=K_(E). There is afirst-order transition in the hysteretic behavior, such that themagnetization reverses in opposite directions during the ascending anddescending branches of the hysteresis loop, see FIG. 2D. At the sametime H_(c) increases by more than a factor of 2, such that H_(c)>K_(U),and H_(E) changes sign and is significantly reduced in magnitude. Uponfurther increasing K_(U), H_(E) vanishes, and H_(c) becomes equal toK_(U), see region III in FIG. 3 as is expected for a coherent rotationmodel without additional unidirectional anisotropy.

It is important to realize that the complexity of this magnetic behavioris due to the noncollinearity of the applied field, the unidirectionalexchange-coupling anisotropy established by the field cooling, and theshape anisotropy determined by the geometry. For example, if theunidirectional anisotropy is parallel to the applied field, then theexchange bias is independent of the shape anisotropy, namely,H_(E)=K_(E)/M_(s), which is exactly the experimental observation, seeFIGS. 1A-1C. It should also be pointed out that the calculatedhysteresis loops do not require that the uniaxial anisotropy be due tothe shape of the ferromagnet. If the ferromagnet has an intrinsicuniaxial anisotropy (i.e., crystalline) then the same effects should beobservable. However, unlike crystalline uniaxial anisotropy, shapeanisotropy introduces an extra degree of freedom, since different partsof the same sample can be easily designed to have different magnitudeand direction of shape anisotropy.

We can estimate, which region of FIG. 3 corresponds to the samples wemeasured. The shape anisotropy of the Fe lines can be calculated fromdemagnetizing factors if one approximates the wires as generalellipsoids. Using M_(s)=1740 emu/cm³ for Fe and the dimensions of 100 mmlength, 300 nm width, and 10 nm thickness results in K_(u)/M_(s)=353 Oe.This compares well with the shape anisotropy determined fromroom-temperature, hard-axis hysteresis loops, which show an anisotropyfield H_(a)˜300 Oe, corresponding to K_(u)/M_(s)˜150 Oe. Theunidirectional exchange-coupling anisotropy can be determined directlyfrom measurements with the field applied along the field coolingdirection FIGS. 1A-1C and is K_(E)/M_(s)=H_(E)=475 Oe. Thus, the samplescorrespond to region I in FIG. 3. Therefore the exchange bias should beequal to K_(u)/M_(s), and in fact the exchange bias in FIGS. 1D and 1Fis ±180 Oe, corresponding well to K_(u)/M_(s)=150 Oe, determined fromthe room-temperature hysteresis loops. Of course, one may notice thatthe simulation in FIG. 2B does not show any hysteresis in contrast tothe experimental data. This is most likely due to the fact that themodel ignores more complicated origins of coercivity in exchange-biassystems, such as irreversible losses in the antiferromagnet. Thesecontributions can be removed from the experimental data by averaging thebranches of the two hysteresis loops and the result is shown by thesolid symbols in FIG. 2B together with the corresponding numericalsimulation. The result is remarkable, since without any free parameter,not only the shift of the loop but also the overall shape of the loopare well described.

In the past, various other approaches have been used successfully tomodify exchange bias locally, for example, by ion irradiation. Onedistinct advantage of the work presented here is that the use of shapeanisotropy provides precise control of the magnitude and orientation(i.e., sign) of the exchange bias over a wide range. This means thatonce the unidirectional exchange-coupling anisotropy is known (i.e.,from an unpatterned film), the coherent rotation model can be used topredict quantitatively the resulting exchange bias shifts of thepatterned areas.

In summary, it is proven that this new patterning technique can be usedto define lateral structures for multilayers resulting in well definedphysical properties. In this particular case it is shown that uniaxialshape anisotropy can give rise to exchange bias in situations where onenaïvely would not expect any. Numerical simulations based on a coherentrotation model show that this effect relies on the noncollinearity ofthe involved anisotropies. The exchange bias is most pronounced when theuniaxial anisotropy is slightly smaller than the unidirectionalexchange-bias anisotropy. Furthermore, as a function of the ratiobetween the uniaxial and the unidirectional anisotropy K_(U)/K_(E), thenumerical simulations provide a phase diagram with three regions ofhysteretic behavior and a change of sign for the exchange bias. Futureexperiments with varying ratios of K_(U)/K_(E) should be able to explorethe full range of predicted hysteretic behavior. Furthermore, thedirectional selectivity of the exchange bias due to shape anisotropy canbe used to establish different preferred magnetization directions inseparately patterned structures with the same magnetic history.Similarly, one can expect that the competition between shape anisotropyand internal interactions in other types of magnetic heterostructurescan give rise to equally rich varieties of magnetic behavior.

In accordance with features of the preferred embodiment, an etch-mask inaccordance with the preferred embodiment mimics layers to be patternedfor producing a pattern transfer.

Referring to FIG. 4, there is shown a lift-off step generally designatedby the reference character 100 for an exemplary etch-mask 101 thatmimics the layers to be patterned in accordance with the preferredembodiment. An etch multilayer step generally designated by thereference character 102 in accordance with the preferred embodimentresults in the complete pattern transfer. The etch multilayer step 102can be an ion-milling operation.

The etch multilayer step 102 for pattern transfer in accordance with thepreferred embodiment also removes the etch-mask 101, thus eliminatingthe need for mask stripping of conventional processes.

Referring now to FIGS. 5-8, there is shown an experimental demonstrationof a method for producing a pattern transfer in accordance with thepreferred embodiment.

Referring first to FIG. 5, there is shown an initial structure generallydesignated by the reference character 500 including an MgOsubstrate(100) 502, Fe/FeF₂ bilayers 504, and a Al layer 506. The Felayer of the Fe/FeF₂ bilayers 504 is capped by the Al layer 506 toprevent oxidation of the Fe layer. As shown, the Al layer 206 is 4 nm,the Fe layer is 10 nm and the FeF₂ layer is 90 nm.

Referring to FIG. 6, there is shown a modified structure generallydesignated by the reference character 600 including a resist layer 602,such as, a PMMA layer for electron beam lithography that is deposited onthe initial structure 500, for example, to define lines having a linewidth, such as, 180-350 nm and a line pitch, such as, 500 nm andcovering an 100×100-μm² area.

Referring to FIG. 7, there is shown a modified structure generallydesignated by the reference character 700 for a lift-off step. Themodified structure 700 includes an etch-mask 702 that is deposited onthe PMMA layer 602 that is removed. The etch-mask 702 includes the samematerials as those to be etched. The etch-mask 702 includes an array oflines 704, each line 704 including an Al layer 706 that is 4 nm, a Felayer 708 that is 10 nm and an Al layer 710 that is 4 nm.

Referring to FIG. 8, there is shown a final structure generallydesignated by the reference character 800 for the etch multilayer step.The modified final structure 800 includes a pair of lines 802 supportedby FeF₂ layer of bilayers 504 carried by the MgO (100) substrate 502.Each line 802 including a Fe layer 804 that is 10 nm and an Al layer 806that is 8 nm, and the FeF₂ bilayers 504 is 90 nm.

In accordance with features of the preferred embodiment, different fielddirections are established on one single chip by using shape anisotropy.In the presented case the patterned systems show still a hystereticbehavior, which would be detrimental for an actual sensor device.Nevertheless, these issues can be easily resolved through a selectedcombination of ferromagnetic and antiferromagnetic materials. Also thecurrent demonstration is made with materials chosen to answer specificbasic science questions. Therefore the exchange bias is only establishedat low temperatures (<78 K), which is clearly undesirable for practicalapplications. However an extension of the present idea toroom-temperature compatible materials is straightforward.

Referring now to FIG. 9, there is shown an exemplary design for a vectormagnetic field sensor generally designated by the reference character100 in accordance with the preferred embodiment. Magnetization of a freelayer is a vector and while ordinary or conventional spin-valves measureonly one component; the vector magnetic field sensor 900 with tworeference directions allow measuring orientation of magnetization vectorrepresented as follows:ΔR₁˜cos (θ−θ₁); andΔR₂˜cos (θ−θ₂)Exchange bias is established in the vector magnetic field sensor 900 byapplying a magnetic field during preparation and annealing afterpreparation in the magnetic field applied along a direction, whichbisects the long axes of the two spin-valves comprising the sensor.Consequently there is one fixed unidirectional anistropy direction,along the applied magnetic field for the whole sample, that is thesingle chip defining the vector magnetic field sensor 100. Combined withthe shape anisotropy, which is uniaxial and is established by geometry,the unidirectional anisotropy will give rise to two separate referencedirections for the pinned magnetization layers in the two sensorcomponents.

The vector magnetic field sensor 900 includes a free layer 902consisting of a soft ferromagnetic metal, for example, such as apermalloy. The vector magnetic field sensor 900 includes a separatinglayer 904 between the pinned (906) and free (902) magnetization layer,such as a non-magnetic metal for a spin valve or an insulator for atunnel junction. The vector magnetic field sensor 900 includes a pinnedlayer 906 for different field directions consisting of a ferromagneticmetal, such as, a CoFe layer, disposed on the separating layer 904. Thevector magnetic field sensor 900 includes a pinning layer 908 fordifferent field directions consisting of an antiferromagnet, such as, aFeMn layer, disposed on the pinned layer 906. A pair of resistances 910of the vector magnetic field sensor 900 are represented by R₁ and R₂.

Referring again to FIG. 8, the modified final structure 800 provides anexemplary demonstration sample for a vector magnetic field sensor 900 inaccordance with the preferred embodiment. The modified final structuredemonstration sample 800 includes the MgO substrate(100) 502, Fe/FeF₂bilayers 504, and Al layer 806. The Fe/FeF₂ bilayers 504 capped by theAl layer 806 prevents oxidation of the Fe layer.

While the present invention has been described with reference to thedetails of the embodiments of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

1-9. (canceled)
 10. A vector magnetic field sensor for measuring anexternal magnetic field comprising: a single chip; said single chiphaving different magnetic reference directions established; and saiddifferent field directions being established on said single chip byusing shape an isotropy.
 11. A vector magnetic field sensor as recitedin claim 10 wherein said single chip includes a free soft ferromagneticlayer.
 12. A vector magnetic field sensor as recited in claim 2 whereinsaid free layer includes a permalloy.
 13. A vector magnetic field sensoras recited in claim 10 wherein said single chip includes a separatinglayer carried by a free layer.
 14. A vector magnetic field sensor asrecited in claim 13 wherein said separating layer includes a spin valvedefined by a non-magnetic metal or tunnel junction defined by aninsulator.
 15. A vector magnetic field sensor as recited in claim 13wherein said separating layer includes a tunnel junction defined by aninsulator.
 16. A vector magnetic field sensor as recited in claim 10wherein said single chip includes a pinned ferromagnetic layer carriedby a separating layer.
 17. A vector magnetic field sensor as recited inclaim 10 wherein said pinned layer includes a CoFe layer.
 18. A vectormagnetic field sensor as recited in claim 10 wherein said single chipincludes a pinning layer carried by a pinned layer.
 19. A vectormagnetic field sensor as recited in claim 18 wherein said pinning layerincludes a FeMn layer.